FIG. 9 is a conceptual diagram of conventional mobile device 7. Proximity alarm device 6 includes control unit 1, amplifying unit 2, and capacitor 3. Loudspeaker unit 4 is electrically connected to an output side of proximity alarm device 6. Proximity alarm device 6 is mounted on mobile device 7.
Control unit 1 receives a vehicle signal and outputs an audio signal. Amplifying unit 2 amplifies the audio signal. An output from amplifying unit 2 is supplied to loudspeaker unit 4 via capacitor 3. Note here that capacitor 3 cuts a DC voltage in the output from amplifying unit 2. With the above-mentioned configuration, proximity alarm device 6 issues alarming sound 8 from loudspeaker unit 4.
Note here that prior art literatures relating to the invention of the present application include, for example, PTL 1.